The diagnostic connector on automobiles permits service personnel to quickly diagnose problems related to the operation of on-board sensors and microcomputers. There is a need, however, for the capability to diagnose individual cylinder performance, that is, to identify misfiring or otherwise under performing cylinders. Such information, in the form of compression pressure and power output values for each cylinder, can be obtained from engine speed variations caused by the compression and power strokes of each cylinder.
The U.S. Pat. No. 4,539,841 to Schroeder et al, entitled "Method of Determining Engine Cylinder Compression Pressure and Power Output", teaches how for a given engine, at a predetermined engine speed and load, the peak compression pressure and the cylinder power output can be closely estimated from instantaneous engine speed at properly selected crank angles. As taught in the patent, however, the method requires a fairly high resolution encoder to generate acceptable instantaneous speed data (e.g., the speed should be measured over a crank angle smaller than about 4.degree.). In practice it means a separate speed sensor has to be used to collect speed data from the ring gear.
It is strongly preferred to obtain the necessary information without a separate speed sensor. Low resolution sensors are commonly used on engines for ignition timing and are capable of delivering a speed sample each 60.degree. or 30.degree., for example.